ARM: mach-mxs: add stmp378x-devb

STMP378x and MX23 are the same and just relabeled. There is a
mach-stmp378x, however, it has a lot of reinvented interfaces, leaking
all sorts of mach-specific functions into the drivers. One example is
the dmaengine which does not use the linux dmaengine-API but some
privately exported symbols. This makes generic use of the drivers
impossible. mach-mxs does it better, so convert the board to mach-mxs.
After that, it is possible to delete all stmp-specific code which should
ease further ARM-consolidation.

Compile tested only due to no hardware (seems not available anymore).

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>